1. Field of the Art
The present invention relates to a method and an apparatus for controlling a hydraulically-operated power transmitting system used in an automotive vehicle, which system includes a continuously variable transmission (hereinafter referred to as "CVT").
2. Related Art Statement
A power transmitting system for a vehicle including a CVT is provided with an auxiliary transmission for widening a range of a speed ratio of the CVT and thereby improving the drivability of the vehicle. For example, a power transmitting system is disclosed in Japanese patent application No. 59-123620 (laid-open under Publication Nos. 61-2957 on Jan. 8, 1986) and 59-149569 (laid-open under Publication No. 61-31752 on Feb. 14, 1986). In such a transmitting system, a speed ratio "r" of the CVT (N.sub.in /N.sub.out, where N.sub.in : input speed N.sub.in of an input shaft of the CVT, and Nout: output speed of an output shaft of the CVT) is controlled so that the input speed N.sub.in coincides with a target input speed N.sub.ino which is determined based on a currently selected position (Drive-range position D, Low-range position L, etc.) of an operator-controlled shift lever.
In a power transmitting system of the type indicated above, however, an amount of change in the speed ratio "r" of the CVT is unnecessarily increased, if the CVT and the auxiliary transmission are operated simultaneously, for example, if the CVT is shifted up during a shift-up action of the auxiliary transmission, or if the CVT is shifted down during a shift-down action of the auxiliary transmission. The unnecessarily increased change in the speed ratio "r" of the CVT leads to a reduction in the driving feel or comfort of the operator of the vehicle.
A simultaneous operation of the CVT and the auxiliary transmission will cause another problem, if a single common hydraulic pressure source is used to operate the CVT and the auxiliary transmission. More specifically, simultaneous shifting actions of the CVT and the auxiliary transmission require a concurrent use of a pressurized working fluid in a comparatively large amount, with a result of a drop in the line pressure of the system, and a consequent slip of the transmission belt of the CVT, which may reduce the operating durability of the transmitting system.